The present disclosure relates generally to systems and methods for optimizing the speed schedule of an aircraft for improved fuel efficiency and aircraft predictability.
A flight management system (FMS) onboard an aircraft is a specialized computer system that automates a wide variety of in-flight tasks. A primary function of a FMS is in-flight management of the flight plan. Using various sensors to determine the aircraft's position and an autopilot system, the FMS can guide the aircraft in accordance with the flight plan. Typically an FMS comprises a navigation database that contains the elements from which the flight plan is constructed. Given the flight plan and the aircraft's position, the FMS calculates the course to follow. The pilot can follow this course manually or the autopilot can be set to follow the course.
The flight plan includes a vertical trajectory, a lateral trajectory, time, and a speed schedule to be followed by the aircraft with respective tolerances, enabling the aircraft to reach its destination. The calculations of the flight plans are based on the characteristics of the aircraft, on the data supplied by the crew and on the environment of the system. The positioning and guidance functions then collaborate in order to enable the aircraft to remain on the trajectories defined by the FMS. The trajectories to be followed are constructed from a succession of “waypoints” associated with various flight points, such as altitude, speed, time, modes, heading, and other points. The term “waypoint” encompasses any point of interest where the point is defined using two, three or four dimensions. A trajectory is constructed from a sequence of segments and curves linking the waypoints in pairs from the departure point to the destination point. A segment or series of segments may be constrained by one or more economic constraints (e.g., time, fuel, and/or cost or a combination thereof). Each segment or series of segments so constrained will be referred to herein as an economic constraint (EC) trajectory segment. The speed schedule represents the speed and speed mode that the aircraft should maintain over time as it flies along the flight trajectory.
In aeronautics, the quantities used to define speed are indicated airspeed, the calibrated airspeed, true airspeed and Mach number. The indicated airspeed (IAS) is the speed corresponding to the speed indicated on the onboard instruments. The calibrated airspeed (CAS) corresponds to the speed after correction is applied to the IAS. The true airspeed (TAS) is the speed relative to the air mass the aircraft is traversing. The Mach number is the ratio of speed to the speed of sound. The value representing speed in a speed schedule can be defined as any of these speeds or can also be a groundspeed. If the time constraint is bound to an Earth-referenced point, the meeting of a time constraint is dependent on any of these speeds translated to a groundspeed, aircraft performance limitations and available distance. The groundspeed is the horizontal component of the speed of the aircraft relative to the ground. More precisely, the groundspeed is equal to the magnitude of the vector sum of the air speed and the wind speed projected onto the horizontal plane. The speed of the aircraft is the vector consisting of the vertical speed and the ground speed of the aircraft.
It is often desirable that the aircraft reach a particular point along the flight path having optimized fuel, cost, and time. Often these three parameters are in conflict with each other, especially in the case when one parameter is specified as a higher priority; yet, no matter which constraint (time, fuel, or cost) has priority, the solution needs to be optimized across the three constraints. Time, fuel and cost are herein referred to as “economic constraints”. An economic constraint can be any one of or a combination of time, fuel and cost constraints. One example of prioritizing economic constraints is the following: if a point has a required or scheduled time constraint associated with it, the optimized solution to reach that point, at the designated time, should still attempt to minimize the fuel and cost. This example might reflect a situation wherein aircraft are being sequenced for arrival at an airport. In yet another example where time is not a constraint, an airline is concerned primarily with restricting costs associated with crew and fuel fees, giving cost a higher priority over other constraints. The solution, in this example, would be to minimize cost to the greatest extent possible but still consider keeping the aircraft on its scheduled arrival time at the destination.
There are multiple ways to alter the aircraft's speed schedule to reach a particular point, such as manipulation of the throttles, yoke, flight plan, economic parameters (cost index, fuel flow factors, and performance factors), speed constraints, speed transitions, speed restrictions or speed modes. This introduces a need for a method and system that can optimally alter the speed schedule of an aircraft along a flight trajectory. This need arises from the ongoing increase in air traffic and the corresponding workload for air traffic controllers and airline operating costs.
In the interest of increased safety and improved airspace or airspace capacity, time constraints are imposed on the aircraft during all flight phases (e.g., departure, climb, cruise, descent and airport approach). This ensures that aircraft arrive at a particular point in their flight plan at a controlled arrival time, scheduled time, constrained time or required time of arrival (hereinafter “RTA”). For example, an RTA waypoint may be a landing runway threshold, an air traffic convergence point, crossing points, etc. Ensuring an aircraft arrives at an RTA waypoint on time may make it possible, for example, to smooth the flow of aircraft before the approach phase and maintain a desired spacing between aircraft.
In the interest of increased economic viability, fuel and cost constraints may be imposed on a portion or all of a flight's trajectory without imposing time constraints. One example of this could be wherein an airline wants to minimize costs or fuel burned for a portion of the flight and air traffic control does not require a time constraint. In this example, an aircraft is provided a speed schedule that meets a cost constraint. Another example is when an air traffic controller is presented with flight trajectory predictions (e.g. estimated flight path, fuel, speed, altitude, and time) which identify that the economic constraints will be met if the flight holds true to the predictions. Should the flight deviate from the flight predictions by more than a predetermined tolerance, an economic constraint with respect to an RTA point could be imposed by the airline or air traffic controller.
The FMS calculates estimated fuel and estimated time of arrival (hereinafter “ETA”) at the RTA waypoint, i.e., the time at which the FMS predicts that the aircraft will arrive at the RTA waypoint. If the ETA departs from the RTA by more than a predetermined tolerance, a new speed command takes place, causing the FMS to redefine the trajectory to be followed by taking account of the time constraint to be observed. The aim is to have the ETA converge with the RTA within a configurable time tolerance (e.g., ±15 seconds). This is accomplished by changing the speed of the aircraft.
Performance optimization allows the FMS to determine the best or most economical speed to fly. This is often called the ECON speed and the corresponding economy speed mode maintains the economy speed. The aircraft's speed while in the economy speed mode is based on an economic optimization criterion called the cost index, the weight of the aircraft, its altitude, wind and the ambient temperature. The cost index is an optimization criterion defined by the ratio of the costs of time and the costs of fuel. As a variant, the optimization criterion may take into account other costs, such as nuisance costs (noises, polluting emissions, etc.).
Current aircraft operations typically employ an RTA function or a fixed speed solution that is commanded to be performed “now”. While an RTA function is active, the aircraft speed will fluctuate as new estimated time predictions are made as a result of groundspeed changes. The groundspeed fluctuates with changes in wind speed. As the aircraft speed fluctuates, the thrust will vary respectively. The RTA functions implemented in today's flight management systems are limited in the current and envisioned mid-term (next 25 years) to a single RTA function capability. The RTA function assigns and allows control to only one waypoint in the flight plan. There is an airline and air traffic operational preference to assign and control to multiple RTAs.
Additionally, some air traffic controllers and pilots are reluctant to assign an RTA to an aircraft or fly the RTA function. The air traffic controllers' reluctance is due, in part, to a potential loss of aircraft separation. In an area where procedural separation is used, such as when aircraft are flying over oceans, a variable speed is unacceptable for separation assurance. During procedural separation, knowledge of the speed, distance, and time (within specified error tolerances) is relied upon for ensuring separation. Also, if the aircraft speed varies, this would require a greater separation distance, which equates to a loss of airspace efficiency. This airspace inefficiency is a loss of airspace capacity, which translates to fewer aircraft in a given amount of airspace. Air traffic controllers may also be reluctant if they are unsure where the RTA mode would be executed and what characteristics the aircraft would follow before and after the RTA waypoint. The pilot's reluctance is due, in part, to the unpredictable nature of the RTA algorithms implemented by the multitude of flight management system manufacturers, each having its own unique behavioral characteristics. The pilots are also sensitive to how the RTA functions in relation to the performance limitations of the aircraft and passenger comfort.
In other instances, air traffic controllers provide a fixed speed command. The fixed speed solution overcomes most of the limitations of the RTA function, but is not optimized for fuel efficiency, is delivered via voice command, is applicable to a single waypoint, and does not provide an automated datalink solution. The fixed speeds are generated to be performed as “now” instructions, which does allow an aircraft to regain the time difference but does not consider optimization, the speed mode or resuming the economy speed mode when the constraint no longer exists. In addition, the use of a fixed speed command inherits delay in communication and time for the pilot and/or the aircraft to reach the specific speed. The controller would then have to “time” delivery of the speed instruction with these limitations in mind to achieve the desired results. Assuming the aircraft is still in the controller's sector, another voice command would have to be given for the aircraft to resume the previous speed and/or speed mode.
There is a need for systems and methods for optimally controlling the speed and speed mode of an aircraft that provide the advantages of the fixed and economy speed and speed modes and avoid the disadvantages of the RTA function.